Baldness happens when a hormone called DHT shrinks your hair follicles until they can no longer produce visible hair. About 20% of men show signs of pattern baldness in their 20s, 30% in their 30s, and roughly 40% by their 40s. The process is primarily genetic, but hormones, stress, nutrition, and lifestyle all play a role in how fast it happens and how far it goes.
How DHT Shrinks Your Hair Follicles
Every hair on your head grows in cycles. The growth phase (called anagen) lasts two to six years, followed by a brief transition period and then a resting phase where the hair eventually falls out and a new one begins growing. This cycle repeats throughout your life, and you normally shed 50 to 100 hairs a day without noticing.
The problem starts with testosterone. An enzyme in your scalp converts testosterone into a more potent form called dihydrotestosterone, or DHT. When DHT binds to receptors on your hair follicles, it triggers them to shrink. Each time a follicle cycles through growth and rest, it comes back slightly smaller. The growth phase gets shorter. The hair it produces becomes thinner, lighter, and shorter. Eventually the follicle miniaturizes to the point where it only produces a fine, nearly invisible hair, or stops producing anything at all.
This process doesn’t happen everywhere on your scalp. The follicles on the top and front of your head are far more sensitive to DHT than those on the sides and back. That’s why the classic horseshoe pattern of remaining hair is so consistent across men who go bald.
Genetics Determine Your Sensitivity
DHT circulates through every man’s body, yet not every man goes bald. The difference comes down to how sensitive your follicles are to the hormone, and that sensitivity is written into your DNA. Research published in The American Journal of Human Genetics identified the androgen receptor gene, located on the X chromosome, as the single most important genetic factor in early-onset baldness. This gene alone accounts for roughly 46% of the risk.
Because the androgen receptor gene sits on the X chromosome, men inherit it from their mothers. This is why the old saying about looking at your mother’s father has some truth to it. But baldness is polygenic, meaning multiple genes contribute. You can inherit risk factors from both sides of your family. Having a bald father still increases your chances, even though the most influential gene comes through the maternal line.
What these genes control, at the molecular level, is how your follicle receptors respond when DHT shows up. Someone with a highly sensitive version of the androgen receptor will start losing hair in their early 20s. Someone with a less reactive version may keep a full head of hair into old age, despite having similar DHT levels.
How Baldness Typically Progresses
Hair loss follows recognizable patterns that dermatologists track using the Norwood Scale, a seven-stage system. In the earliest stage, there’s no noticeable recession. The first visible sign is usually a receding hairline at the temples, forming an M shape. Some men instead notice thinning at the crown first. Over time, both areas of loss expand and eventually merge, leaving only a band of hair around the sides and back of the head in the most advanced stage.
The speed of this progression varies enormously. Some men move from early recession to significant baldness in under a decade, while others stay at a mild stage for 20 or 30 years. There’s no reliable way to predict the pace, though earlier onset generally signals a more aggressive pattern.
Inflammation Plays a Larger Role Than Expected
Doctors have traditionally classified pattern baldness as a non-inflammatory condition, distinguishing it from scarring forms of hair loss. But research in the Journal of the American Academy of Dermatology tells a more complicated story. When pathologists examined scalp biopsies from people with pattern baldness, they found inflammation around the follicles in 86% of cases. The inflammation was typically mild, concentrated in the upper portions of the follicle rather than at the base.
This low-grade, chronic inflammation appears to work alongside DHT. As follicles shrink, the surrounding tissue becomes irritated, which may accelerate the miniaturization process. Over many years, this can lead to subtle scarring around the follicle that makes the damage harder to reverse. It’s one reason why treating hair loss earlier tends to produce better results than waiting.
Why Stress Makes Hair Fall Out
Stress-related hair loss works through an entirely different mechanism than pattern baldness, though the two can overlap. Research from Harvard’s Stem Cell Institute revealed the specific pathway: when you’re under chronic stress, your adrenal glands pump out cortisol. That cortisol acts on a cluster of cells beneath each hair follicle called the dermal papilla. Normally, these cells release a signaling molecule that tells hair follicle stem cells to wake up and start growing new hair. Cortisol blocks that signal.
The result is that follicles get stuck in their resting phase. Instead of the normal 50 to 100 hairs falling out daily, you might lose several hundred, and new growth stalls. This type of shedding, called telogen effluvium, usually becomes noticeable two to three months after a major stressor. The good news is that it’s typically reversible once the stress resolves, since the follicles themselves aren’t damaged.
Nutritional Deficiencies That Trigger Shedding
Your hair follicles are among the most metabolically active cells in your body, and they need a steady supply of nutrients to keep cycling. Two deficiencies stand out in the research: iron and vitamin D. A study of women with pattern hair loss found that 89% had deficient or insufficient vitamin D levels (below 30 ng/ml) and 74% had low iron stores. While the relationship isn’t perfectly straightforward, correcting these deficiencies often slows shedding in people whose levels are low.
Iron is especially important because your follicles need it to produce the proteins that make up the hair shaft. Ferritin, the protein that stores iron in your body, ideally sits at 70 µg/L or above for healthy hair growth. Many people, particularly women with heavy periods or anyone on a restricted diet, fall well below that threshold without realizing it.
Smoking and Oxidative Stress
If you’re genetically predisposed to baldness, smoking appears to accelerate the timeline significantly. A 2020 study comparing 500 male smokers and 500 nonsmokers between ages 20 and 35 found that 85% of smokers showed some degree of hair loss, compared to 40% of nonsmokers. The severity gap was even more dramatic: 47% of smokers had reached advanced stages of loss, versus just 10% of nonsmokers.
The mechanism involves oxidative stress. Tobacco smoke floods your body with free radicals, reactive molecules that damage cell DNA. Hair follicles that are already shrinking from DHT exposure are particularly vulnerable to this additional damage. A 2018 review confirmed that cells in balding areas of the scalp are more sensitive to oxidative stress than those in non-balding areas, creating a vicious cycle where genetic susceptibility and environmental damage reinforce each other.
How Women Lose Hair Differently
Women experience pattern hair loss too, but it looks different and likely involves different triggers. Instead of a receding hairline, women typically see diffuse thinning across the top of the scalp, with the hairline staying intact. The part gradually widens, and the overall volume decreases. Complete baldness is rare in women.
The hormonal connection is murkier. While androgens like DHT clearly drive male pattern baldness, the link between androgens and female hair loss remains unclear. Genetics still play a role, and hormonal shifts during menopause often trigger or worsen thinning, but the biology isn’t as well understood as it is in men.
What Treatments Can Actually Do
The two most established treatments work by targeting different parts of the process. One blocks the enzyme that converts testosterone to DHT, reducing the hormone that causes follicle shrinkage. The other increases blood flow to the scalp and extends the growth phase of the hair cycle. Neither is a cure, and both require continuous use to maintain results.
Long-term data is encouraging for people who stick with treatment. A 10-year Japanese study of over 500 men found that 99% maintained their hair or improved, with the average participant gaining back roughly one grade on the baldness scale. Results were strongest in men who started treatment earlier, when follicles were miniaturized but not yet dead. Once a follicle is completely gone, no medication can bring it back, which is the fundamental limit of pharmaceutical treatment.
Hair transplantation works around this limit by relocating DHT-resistant follicles from the back and sides of the scalp to thinning areas. These transplanted follicles retain their genetic resistance, which is why transplanted hair typically lasts permanently even as native hair continues to thin around it.